This invention relates to red light emitting phosphors capable of emitting red light upon excitation with long-wavelength ultraviolet radiation or short-wavelength visible light of 350 to 420 nm, methods for preparing the same, and light emitting devices comprising the read light emitting phosphors.
Light emitting diodes (LEDs) are semiconductor light emitters adapted to produce light by converting electrical energy into ultraviolet light, visible light, infrared light or the like. For example, those emitters utilizing visible light are semiconductor light emitters formed from light emitting materials such as GaP, GaAsP and GaAlAs, and LED lamps having such emitters encapsulated with transparent resins are widely used. Also LED lamps of the display type are often used in which light emitting materials are secured to an upper surface of a printed circuit board or metal leads and encased in a transparent resin casing on which numerical figures or characters are configured.
LEDs have a long lifetime and high reliability because of semiconductor devices, and facilitate replacement operation when used as light sources. LEDs are thus widely used as components in portable communications equipment, personal computer peripheral equipment, business machines, household electric appliances, audio equipment, switches, back light sources, and display device such as bulletin boards.
The color of light emitted by the LED lamps can be altered by introducing various powder phosphors into transparent resins with which semiconductor light emitters are encapsulated. Depending on the intended application, any color in a wide spectrum in the visible region from blue to red is available.
However, recently users increasingly impose a more stringent demand on the color of such various display device, requiring display devices to have an ability to precisely reproduce a subtle color tone. It is strongly required that a single LED lamp emit white or any desired intermediate color light.
Attempts were then made to display white or any desired intermediate color with a single LED lamp by applying various red, green and blue phosphors to the surface of the semiconductor light emitter in the LED lamp, or incorporating such various phosphors in the encapsulant or coating material of the LED lamp.
Among these phosphors, currently used typical examples of those phosphors to be excited with long-wavelength ultraviolet radiation or short-wavelength visible light (350 to 420 nm) include BaMg2Al16O27:Eu and (Sr,Ca,Ba)5(PO4)3Cl:Eu for emission of blue light, BaMg2Al16O27:Eu,Mn and Zn2GeO4:Mn for emission of green light, and Y2O2S:Eu, La2O2S:Eu and 3.5MgO·0.5MgF2·GeO2:Mn for emission of red light. Using appropriate ones of these light emitting phosphors, an emission color over a wide range is available.
However, the above-noted red light emitting phosphors have the problem of weak light emission to long-wavelength ultraviolet radiation and short-wavelength visible light (350 to 420 nm), as compared with the blue and green light emitting phosphors.
Then, when whitish light is produced using lights of these wavelengths, the proportion of the red light emitting phosphor must be increased at the sacrifice of cost. Since white color of light is obtainable by balancing the quantities of red, green and blue light emissions, the quantities of green and blue light emissions must be reduced in proportion to the quantity of red light emission in order to produce emission of a whitish light. Further, an upper limit is imposed on the amount of phosphors used. As a result, a less quantity of white light emission is available, giving rise to problems including a failure to produce white light with a high luminance.
The wavelength corresponding to the excitation energy of electron pairs oxide base compounds possess is within the ultraviolet region, and the wavelength of long-wavelength ultraviolet radiation and short-wavelength visible light (350 to 420 nm) overlies the absorption edge of phosphors. Then, particularly for red light emitting phosphors, there arises the problem that the quantity of light emission of a phosphor changes significantly if the peak of emission wavelength of a semiconductor light emitter varies.
To solve this problem, rare earth oxysulfide phosphors activated with europium are proposed in JP-A 11-246857, JP-A 2000-144130, etc. It is reported that the excitation wavelength for these phosphors is shifted to the longer wavelength side.
On a longer than 350 nm wavelength side, however, the absorption intensity of these red light emitting phosphors shows a sharp decline as the wavelength becomes longer. When a light source with an emission peak at 350 to 420 nm, for example, a ultraviolet LED is used as an excitation light source, the quantity of emission of a phosphor changes significantly due to variations of the wavelength of LED emission, that is, the wavelength of excitation light, which inevitably occur for manufacturing reasons. This results in variations of color tone when the above phosphor is used in combination with green and blue light emitting phosphors to produce white or intermediate color. The conventional red light emitting phosphors are difficult to precisely reproduce a subtle color tone.